This invention is concerned with improvements in and relating to apparatus and methods for materials investigations. The invention is particularly, but not exclusively, concerned with investigating gamma ray emissions from materials. The invention is still more particularly, but not exclusively, concerned with correction techniques in such investigations and the provision of a new correction technique.
In a variety of situations it is necessary to investigate emissions from radioactive sources in or on materials to form a basis for a variety of subsequent decisions, actions or further considerations. The investigations of the samples may relate directly to the emission, for instance the emission source, or indirectly, for instance the consideration of associated non-emitters or emitters which are not directly measurable. The emissions of interest are in particular gamma ray emissions, but other emission forms may be considered additionally or alternatively.
Emission investigation is particularly important in waste evaluation cases. For a given waste sample it is desirable to be able to determine a variety of unknowns. The unknowns may include, but are not limited to, one or more of the level, type, constituents, nature and distribution of the emissions, emission sources, associated materials or associated factors.
When taking measurements of the radioactive waste contained in a sample using prior art techniques it is necessary to make a number of assumptions to gain a solvable system. One such assumption for certain correction techniques is that the materials are homogeneously distributed within a body or container and, in particular that the density profile of the material within the container is even. Given the various materials encountered and the varying size and shape of the materials potentially making up the waste this is not a truly valid assumption. Further variations occur in practice as materials settle or move over time, for instance during transportation.
Relevant correction techniques accept this assumption and are qualified by an error level as a result. For safeguard reasons the level of radioactive sources in a sample should be determined as accurately as possible. In safety cases the error requires that the level be placed at the worse case level. As a consequence the level of radioactive sources is frequently over estimated, with consequential complexity of further handling or storage, as well as cost penalties.
The present invention seeks, amongst other aims, to provide a technique in which greater account of variations or potential variations in density are taken. The improved account may be used in accounting for the effects of the material on the emissions as detected.
According to a first aspect of the invention we provide a method of investigating radioactive sources in a body of material provided at an investigation location, the body of material comprising a plurality of samples, the method comprising detecting a portion of the emissions arising from a sample, the detected portion relating to a detected level, the detected level being corrected according to a correction method to give a corrected level, the method being repeated for one or more of the other samples, the correction method for one or more of the samples comprising providing a generator of radioactive emissions and detecting the radioactive emissions from the generator with the sample at the investigating location, the relationship of the emissions detected with the sample at the investigating location to the emissions which would be detected with the sample absent from the investigating location determining a characteristic of the sample, the determined characteristic being employed as a factor in the correction method used for that sample to obtain the corrected level.
The body of material may be free standing, but is preferably contained in a container. The sample may include the part of the container associated with that part of the body of material.
Preferably the sample is a portion of the body of material which extends from one side of the body through to the other. The sample may be a segment or slice through a body of material, preferably a segment or slice which extends outwards to the limits of the body of material in two dimensions. Preferably the sample is taken horizontally through the body of material. Preferably the sample has substantially the same thickness throughout the body of material. Preferably the thickness of the sample is its depth.
The samples may be investigated in order, for instance from one end of the body of material to the other.
Preferably the generator of emissions is a radioactive source provided externally of the position occupied by the container and/or body of material in use. Preferably the generator is provided in opposition to the detectors therefore.
The relationship of the emissions detected with the sample at the investigating location to the emissions which would be detected with the sample absent the investigating location may be the ratio of the respective count rates for the detectors. Preferably the emissions leaving the generator count rate is determined in the absence of the sample. The absence may be an absence of any body of material in the investigating location. The relationship may be based on the ratio R/Ro, where R is the rate at which the emissions are detected with the sample in place and Ro is the rate of emissions which would be detected without the sample in place. Preferably the relationship is based on, and ideally equates to xe2x88x92ln(R/o).
Preferably the characteristic determined is a function of the density of the sample and preferably the density of the sample. The characteristic may be a function of the effective amount of material in the sample. The characteristic may relate to the effective amount of material determined to be in a sample relative to one or more other samples.
The determination of the characteristic for one or more or all of the samples may be based on the interrelationship of a plurality of potential variables. Preferably the variables include one or more of, and most preferably all of:
I) the relationship of the emissions which would be detected without the sample present to the emissions detected with the sample in place, more preferably the relationship of R and Ro, where R is the rate at which the emissions are detected with the sample in place and Ro is the rate of emissions which would be detected without the sample in place;
ii) the attenuation effects of the sample, more preferably xcexc, where xcexc is the mass attenuation coefficient;
iii) the path of the emissions through the sample, more particularly, x, where x is the thickness of the sample (between source and detector);
iv) the density of the sample, xcfx81.
Preferably the determination of the characteristic is based on the equation:
R=Ro exp(xe2x88x92xcexcxcfx81x)
where the symbols have the meanings referred to above.
Preferably under the conditions of the investigation x and/or xcexc, and most preferably both are substantially constant. Preferably x is kept constant by fixed relative generator, detector and sample positions (the sample may rotate without affecting this). Preferably xcexc is substantially constant due to the energy of the generator emissions. An energy of greater than 400 keV, preferably greater than 1000 keV and ideally greater than 1300 keV may be used. Preferably the source is as detailed in the British Nuclear Fuels PLC UK Patent Application no. 9900449.1 filed 11 Jan. 1999 and referenced P17454, the contents of which are incorporated by reference. In particular we may provide a method of investigating radioactive sources in a sample, the method comprising detecting a portion of the emissions arising from the sample, and further comprising the provision of a radioactive generator, passing at least a portion of the emissions of the generator into the sample, detecting at least a portion of the emissions from the generator leaving the sample, the radioactive generator emissions being of at least a plurality of emission energies and at least two of those energies being detected.
Preferably the method further provides that the detected portion of the source emissions relate to a detected level for the sources in a sample, the detected level being corrected according to a correction method to give a corrected level for the sources in a sample, the process being repeated for one or more other samples.
Preferably the correction method employs measured transmission coefficients in determining the correction. The measured transmission coefficients, for one or more of the energies, most preferably all, may be provided according to the equation:       Trans    .          xe2x80x83        ⁢    Coeff    .    =      R          R      o      
where R is the rate of detected photons with the sample in place, Ro is the rate of photons which would be detected without the sample in place.
Preferably the density determined is used as a factor in the correction method. The density used in the correction method may be an averaged density from the determinations or a weighted average density from the determinations.
For correction of source emission energies corresponding to a generator energy preferably the measurement based correction factor is used. For correction of source emission energies not corresponding to a generator energy preferably the correction factor is based on the extrapolation of the correction factors based on the measurements.
The generator is preferably a single isotope. Preferably the emission energies extend across a substantial portion of the range of energies emitted from the sample. A substantial portion may be 50%, preferably 75%, more preferably 90% and ideally 100% of the sample energies range. The generator most preferably of all emits energies encompassing the range of energies emitted by the sample. 152Eu is a particularly preferred generator. Preferably at least 5 energies from the source are detected and used, more preferably at least 8 energies are detected and used.
Preferably the portion of generator emissions detected have passed through the sample. Preferably the generator is provided on the opposing side of the sample to the detectors, most preferably in direct opposition.
One or more of the detectors for the sources may be used for detecting the generator emissions and/or vice-versa.
We may also provide apparatus for investigating radioactive sources in a sample, the apparatus comprising:
one or more detectors for emissions from the sources, the detectors generating signals indicative of the emissions detected;
an investigating location into which the sample is introduced;
signal processing means for relating the detector signals to one or more characteristics of the sources;
a radioactive emission generator separate from the sample; and
one or more detectors for emissions from the radioactive generator leaving the sample;
wherein the radioactive generator emissions are of at least a plurality of energies and a least two of the plurality of energies are detected.
The source detectors and the generator detectors may be one and the same in the case of one or more or all of the detectors.
Preferably the amount of material in a sample is a function of the effect on transmission of generator emissions by that sample, the total amount of material being proportional to the effects of all the samples, the fraction of the total material in a particular sample being a function, preferably a ratio, of that sample""s effect on transmission to the sum of all the effects. The effect on transmission of a sample may be given a numerical value, the amount of material in that sample, and/or its mass, being defined as the numerical value for that sample divided by the sum of the numerical values for all the samples, multiplied by the total mass. A density value for each sample may be made in this way by virtue of the known volume of the sample.
Preferably the amount of material in a sample, Vs, is made proportional to the ratio of R to Ro, and more preferably is based on, and ideally equates to: xe2x88x92ln(R/Ro). In this way the total amount of material in the body of material is proportional to the sum of each sample amount, xcexa3Vs. Preferably the fraction of the body of material in a given sample is Vs/xcexa3Vs.
Preferably the characteristic is determined based on a number of variables including one or more of, and ideally all of,:
I) the total mass of the body of material, M;
ii) the total volume of the body of material, V;
iii) the total number of samples forming the body of material, N;
iv) the amount of material in a given sample, Vs;
v) the sum of all the values proportional to the amounts of material in the sample volumes, xcexa3Vs;
vi) the density of the sample.
Preferably the characteristic is determined based on the equation:
xcfx81=(N.M/V)xc2x7(Vs/xcexa3Vs)
Preferably the characteristic, and particularly the density is used in correcting the detected level to the corrected level, for instance by established subsequent techniques, such as those set out in the Los Alamos primer, 2nd Edition, March 1991, ISBNO-16-032724-5.
According to a second aspect of the invention we provide apparatus for investigating radioactive sources in a body of material, the body of material comprising a plurality of samples, the apparatus comprising:
one or more detectors for emissions from the sources, the detectors generating signals indicative of the emissions detected;
an investigating location into which the sample is introduced;
signal processing means for relating the detector signals to a detected level for the sources;
processing means providing a correction method for correcting the detected level for the sources to give a corrected level;
the apparatus further comprising:
a generator of radioactive emissions, at least of portion of the emissions entering the investigating location and, in use the sample;
one or more detectors for detecting the generator emissions with the sample at the investigating location;
processing means for determining a characteristic of the sample based on the relationship of emissions detected with the sample at the investigating location to emissions which would be detected with the sample absent, the characteristic being employed by the processing means as a factor in the correction method used for that sample to obtain the corrected level.
The source emission detectors and the generator emission detectors may be the same in one or more or all cases.
The signal processing means and/or processing means for the correction method and/or processing means for the determined characteristic may be one and the same.
The second aspect of the invention may include any of the features, options and possibilities set out on the first aspect of the invention, including apparatus suitable for the implementation of the method steps detailed therein.
The first and/or second aspects of the invention may further include any of the features, options, possibilities and steps set out below.
The sources may be singular or plural in disposition and/or type. The sources may be one or more isotopes of one or more elements. The sources may be alpha and/or beta and/or gamma emitters, but are preferably gamma emitters at least.
One or more sources of the same type and/or of different types may be present in the sample. The sources may be homogeneously distributed, or more usually, unevenly distributed. The size and/or shape and/or mass of a source may be different from the size and/or shape and/or mass of another source in the sample, be they of the same or different types.
The sources may be investigated by detecting one or more of their emitting energies. Thus a characteristic energy of an isotope may be detected.
The sources may be investigated directly, for instance they contribute directly to the detected level, and/or the sources may be investigated indirectly, for instance they do not contribute directly to the detected level but are associated with sources which do.
The samples may be gaseous and/or liquid and/or solid. The samples may contain one or more non-emitting or non-source materials. The materials may include one or more of metals, such as iron, steel, aluminum; wood; glass; plastics, such as Polythene, PVC; liquids, such as water.
The container preferably entirely encloses the body of material. The container may be of metal or of concrete or a combination of such materials. Drums are a particularly preferred container, such as right cylindrical drums.
The containers may be of one or more standard sizes. The height and/or diameter of the containers may be standard.
Preferably the containers introduced to the investigating location one at a time. The containers may be introduced by conveying along a surface, preferably a horizontal surface. The surface may include or be formed of a plurality of rollers. Preferably the container is removed from the investigating location in a manner equivalent to its, introduction.
The investigating location is preferably provided in proximity to the emission detector or detectors. The investigating location may be provided in proximity to one or more radioactive sources. The sources are preferably intended to transmit radiation through the sample. Ideally the investigating location is provided between the detector(s) and the transmission source(s).
The sample, preferably the container for it, may be rotated at the investigating location. Preferably the rotation presents different portions of the sample in proximity to the detectors and/or transmission source(s), the rotation may be continuous or stepped. The rotation may be provided at between 5 and 25 rpm.
Preferably the sample and/or body of material and/or container are weighed at the investigating location, for instance by the turntable used to rotate it.
The sample, preferably the container for it, may be raised and/or lowered at the investigating location. Preferably the rasing and/or lowering presents different portions of the sample to the detector(s) and/or transmission source(s), the raising and/or lower may be continuous or stepped. Preferably investigations are performed as the sample is lower and raised.
The sample may be rotated and/or lower and/or raised.
A single detector may be used. Preferably a plurality of detectors, for instance three, may be used.
The detectors may be of the high purity germanium type.
Preferably the detectors are collimated to restrict their field of view to the body of material of which the sample is the whole or a portion thereof. Where the sample is less than the whole of the body of material, preferably the detectors are collimated to restrict their field of view to the sample only. The sample is preferably a slice or segment of the whole. The segments may be of the same thickness.
Preferably the detected level is obtained from a passive counting stage. Preferably the transmission based investigations are performed before and/or after the passive count stage.
Preferably the transmission source(s) is provided in opposition to the detector(s). Preferably the same number of transmission sources are provided as there are detectors. It is particularly preferred that the transmission source be provided according to the nature of the transmission source detailed in British Nuclear Fuels PLC UK Patent Application no. 9900449.1 detailed reference P17454 filed on 11 Jan. 1999 and as detailed above.
One or more surface dosimeters may be provided. Preferably the surface dosimeters are configured to investigate gamma emitting sources. Alpha and/or beta emitting sources may alternatively or additionally be investigated.